In the recording apparatus for performing the recording to a recording medium such as paper or an OHP sheet, there has been proposed the recording apparatuses on which various types of recording heads are mounted. A wire-dot type, a thermal type, a thermal transfer type, and an inkjet type can be cited as an example of the recording heads. Particularly, since the inkjet type of recording is performed by directly ejecting the ink to recording paper, its running cost is low and its recording operation is quiet. Accordingly, it has attracted a great deal of attention.
In the inkjet recording apparatus, a multiplicity of nozzles (ejection ports or recording elements) for ejecting the ink droplets are formed in the recording head. The inside of the nozzles are filled with the ink for performing the recording to the recording medium. When characters, images, and the like are recorded, the recording is performed by selecting appropriately the corresponding nozzles to recording data (image data) from the nozzles to eject the ink. The method of converting heat, energy of a heater provided in the nozzles into ejection energy or the method of converting mechanical energy of an element generating vibration into the ejection energy can be cited as an example of the methods for ejecting the ink.
The inkjet recording apparatus includes a serial scan type (carriage scan type) inkjet recording apparatus and a multi-scan type inkjet recording apparatus. In the serial scan type inkjet recording apparatus, the carriage on which the recording head is mounted performs the recording while reciprocating in a direction substantially perpendicular to the direction in which the nozzles are arrayed in the recording head. In the multi-scan type recording apparatus, the recording is performed by using the recording head having a width substantially equal to the width of the recording medium. The serial scan type inkjet recording apparatus has a configuration in which, after the multiplicity of nozzles included in the recording head are driven based on recording information by the carriage scan to perform the recording of one scan recording area, the recording medium is relatively conveyed by a predetermined amount in the direction substantially perpendicular to the carriage reciprocating direction. The given image is formed by alternately performing the recording scan and the conveyance of the recording medium. In the multi-scan type recording apparatus, the image is formed by performing the recording while the recording medium is conveyed in the direction substantially perpendicular to a nozzle array disposed in the recording head.
Recently, as the inkjet recording apparatus becomes widespread, the high quality of the output image is required. In order to improve the image quality, it is effective to decrease graininess. Therefore, there has been proposed the technology which decreases the ink droplet ejected from the recording head to finely dispose the ink on the recording medium.
In order to finely dispose the ink on the recording medium, it is contemplated to increase a drive frequency of the recording head to shorten an interval of the ink ejection, or it is thought to array the nozzles in high density in the recording head. However, when the drive frequency of the recording head is excessively increased, after the ink droplet is ejected from the recording head, the next ink droplet cannot be ejected because the ink supply cannot catch up. Therefore, when it is desirable that the ink is disposed on the recording medium in a finer manner than the drive frequency determined by the configuration of the recording head, it is realized by the technology that the recording scan is performed at the ejectable drive frequency determined by the configuration of the recording head and the plural-time recording scans of the recording head are performed over the same recording area. In this case, it is necessary that the previous recording scan differs from the subsequent recording scan in ejection timing of the ink droplet from the recording head so that impact positions of the ink droplets are different from each other. However, while the ink can be finely disposed on the recording medium by performing the plural-time recording scans over the same recording area, throughput is decreased.
In order to solve the problem, there is the method in which the recording head which can eject both the ink droplet having a relatively larger volume and the ink droplet having a relatively smaller volume is used, the larger ink droplets are coarsely disposed on the recording medium in the case where the high-speed recording is required rather than the image quality, such as a case in text data, and the smaller ink droplets are finely disposed on the recording medium in the case where the image quality is required rather than the recording speed, such as a case in image data (e.g. a photograph), and thereby both the high-speed recording and the high-quality image become mutually compatible (for example, Japanese Patent Application Laid-Open No. 2002-086760). The method of changing the energy given to the ink during the ejection by controlling current or voltage applied to a heater or a piezoelectric element, or the method of arranging the nozzles for ejecting the relatively larger ink droplet and the nozzles for ejecting the relatively smaller ink droplet in the recording head can be cited as an example of the method of ejecting the ink droplets having the different sizes from the recording head.
(First Problem)
However, when the amount of shift of the impact position of the larger ink droplet is equal to that of the impact position of the smaller ink droplet on the recording medium, degradation of the image quality becomes conspicuous in the recording in which the smaller ink droplets are disposed in high density on the recording medium when compared with the recording in which the larger ink droplets are disposed in low density.
When the recording medium is conveyed while supported by both a conveying roller and a paper discharge roller, the recording medium can be conveyed with high accuracy. However, conveyance accuracy is decreased when the recording medium is conveyed while supported by only one of the conveying roller and the paper discharge roller, that is, immediately after paper feed of the recording medium or immediately before paper discharge. Specifically, a front end area in advance of arrival of a front end of the recording medium at the paper discharge roller and a rear end area subsequent to separation of a rear end of the recording medium from the conveying roller are the area where the conveyance accuracy is decreased. Therefore, the impact positions of the ejected ink droplets are shifted in each of the front end area and the rear end area of the recording medium, which results in the degradation of the image quality.
Accordingly, in the front end and rear end areas of the recording medium where the conveyance accuracy is decreased, when the printing is performed while the smaller ink droplets are disposed in high density, there is the problem that the image quality failure such as unevenness is particularly easy to occur.
(Second Problem)
In the image of the inkjet recording apparatus, as the resolution and quality are improved, a range of application is increased and various modes of forming the image on the recording medium are demanded. For example, when the image taken by a digital camera or the like is output with the inkjet recording apparatus, the output result similar to the photograph output to a photographic paper, i.e. the so-called frameless recording is performed by recording the image on the whole surface of the recording medium without providing a margin in a peripheral portion of the recording medium. The frameless recording is realized by ejecting the ink droplets in the range wider than the recording medium. The inside of the recording apparatus is prevented from becoming soiled by providing a member for absorbing the ink (ink absorber), at the position in the area outside the recording medium, where the ink is ejected.
Sometimes the ink droplet does not reach the surface of the recording medium or the surface of the ink absorber but become mist to diffuse inside the recording apparatus by evaporating the ejected ink droplet before the ink droplet reaches the recording medium or the ink absorber. Because the distance from the nozzle surface of the recording head to the surface of the ink absorber is longer than the distance from the nozzle surface of the recording head to the surface of the recording medium, the mist is easy to generate when the ink droplet is ejected in the area outside the recording medium. Further, because heat capacity is smaller in the ink droplet having the smaller volume, the ink droplet having the smaller volume is easy to evaporate when compared with the ink droplet having the larger volume, and the generation of the mist is increased when the recording is performed with the ink droplet having the smaller volume.
Therefore, when the recording is performed with the ink droplet having the smaller volume in the area outside the recording medium, the mist is easiest to generate. When the mist diffuses inside the recording apparatus, the recording medium becomes soiled by the mist adhering to the conveying roller or the operation of the carriage is obstructed by the mist adhering to a guide shaft. Further, when an optical encoder is used in order to control the moving speed and the position of the carriage and the recording medium, because the ink mist cut off light, the normal control can not be performed. Therefore, sometimes various problems are generated in the recording apparatus such that the speed control or stop position control becomes abnormal.